I think I've found the problem: compizconfig-backend-gconf 0.8.2-1 has not yet migrated to testing. Bizarrely, there is no dependency mentioned, and yet this was the entire problem: upgrading this package from 0.7.6-1 to 0.8.2-1 solved the problem for me. So there is a dependency or conflict missing somewhere, but I'm unsure exactly where it should be. Julian